Settling process analysis device and method

ABSTRACT

A system and method for monitoring the dynamics of particle motion in a liquid-solid media including the rate of settling of particles, the identification of unsettled particle clouds, and the identification and control of the bed level of settled particles in a slurry within a settler is provided. The system includes an ultrasound transducer and a receiver for detecting echoes from particles in the slurry. The echoes are processed to determine the bed level of the settled particles, the position of unsettled particle clouds, and the rate of settling of the particle clouds.

This application is a divisional of U.S. application Ser. No. 08/789,438filed Jan. 27, 1997 now U.S. Pat. No. 5,939,622, which is in turn acontinuation-in-part of U.S. application Ser. No. 08/717,876 filed Sep.23, 1996, now U.S. Pat. No. 5,789,676, which is in turn acontinuation-in-part of U.S. application Ser. No. 08/233,178 filed Apr.26, 1994, now U.S. Pat. No. 5,635,632.

BACKGROUND OF THE INVENTION

Accurate determination of the bed level of settled particles isimportant to control settling tanks such as clarifiers, thickeners, andaluminum and coal settlers. During operation of a settler, a slurryformed of a liquid laden with particles such as aluminum, solid wastesor coal is carried via a pipe to the center top, i.e., the "center well"of a settling tank. The slurry then is discharged into the center well,and the particles are allowed to settle onto the bottom of the tank. Thesettled particles form a bed, and the liquid-solid interface between thewaste suspension and the liquid above is the bed level. Often, however,the particles in the slurry do not immediately settle to the bottom, butremain suspended in the tank to form a "cloud" that can create or be apredictor of an upset condition. Knowledge of the location of bed leveland/or cloud dimensions and intensity is important for controlling theamount of chemicals or flocculants added to the tank to control thesettling process.

Settling rates in thickeners and clarifiers have been manuallycontrolled due to lack of reliable measurement instrumentation. Settlingrates have been determined manually by depositing a sample of the slurrytaken from the settling tank into a graduated cylinder, and employingphoto cells or visual observation to measure the time for the suspendedparticles to fall a given distance within the cylinder. This method,although simple and inexpensive, is not a reliable means of measuringsettling rates since it depends on obtaining a representative samplefrom the settler, is performed outside the settler, and its accuracydepends upon a human observer.

The bed level of settled particles also has been determined by usingsimple non-coherent fish finder (A-Mode) ultrasound systems. In anA-mode system, a transducer sends an ultrasound pulse into the particlesuspension contained in a "settler". Low level echoes return from thesurface of the settled bed. If the speed of the ultrasound pulse isknown, then the distance to the bed level of settled particles can becalculated from the time between the transmitted pulse and the returnedecho by using the well known range equation:

    d=ct/2

where

d=distance to the target,

c=speed of sound in the liquid or other media

t=round trip time from the transducer pulse to echo return.

The simple A-mode systems of the art are useful when a distinct bedlevel boundary exists and where that boundary is essentiallyperpendicular to the ultrasound transducer path. However, if the bedlevel boundary is not nearly perpendicular to the transducer path, orwhen there is no distinct bed level boundary, then echoes from the bedlevel may be blurred or undetectable. Moreover, in the unsettledparticles where a bed level might exist, A-mode ultrasound systemsprovide little or no information on settler performance.

Currently, non-coherent A-mode systems cannot reliably detect bed level,cloud layer and cloud layer characteristics. A need therefore exists forreliable and accurate determination of bed level and cloud layerexistence and particle activity within the cloud layer.

SUMMARY OF THE INVENTION

The invention provides a system and method for use in detection andcontrol of the bed level of settled particles in a slurry. The inventionalso may be used to control the settling rate of particles in slurries.The system includes an ultrasound transducer for transmitting ultrasoundpulses into a slurry within a settling tank. The system also employs apreamp-receiver to detect echoes from particles in the slurry. Theseechoes are processed to determine the bed level of the settledparticles, the existence of clouds and the activity of particles in thecloud in the settler, as well as the settling rate of the particles. Thebed level and settling rate may be used to control addition ofchemicals, slurry additions and the like to the settling tank.

The system employed in the invention can operate in any of the followingcoherent modes: (i) the peak method (coherent A-mode) to detect the bedlevel when it exists, (ii) the moving target detection mode to detectparticle clouds and to assess their characteristics and also to detectthe bed level, and (iii) the Doppler processing mode to detect ascendingand descending particle speed in the liquid suspension in the settlingtank. The peak method and the moving target detection modes are calledthe image modes.

In another aspect, the invention provides a system and method forlocating the bed level of settled particles within a liquid slurry. Themethods comprise transmitting ultrasonic sound waves of a firstfrequency from a transducer into a slurry that has a bed of settledparticles and a cloud of settling particles. The ultrasonic wavesgenerate echoes from the bed and echoes from the settling particles.Digital and analog electrical signals are generated from the echoes, andthe electrical signals are processed to characterize the bed level andthe settling of the particles.

In another aspect, the invention provides a system for identifying bedlevel and the settling particles. The system employs a transducer forsending ultrasound signals into a slurry. The ultrasound signals arereflected as echoes which are captured by a preamp-receiver. Thepreamp-receiver converts the echoes to analog electrical signals. Thesesignals are directed to individual sine and cos channels where thosesignals are multiplied with either sine or cos signal in a mixer andfiltered to remove high frequency products of the mixer. An analogswitch multiplexes the resulting new analog signals to an analog-digitalconverter which converts those analog signals to digital signals. Thedigital signals are stored in a data acquisition memory for numericalprocessing according to any of the peak method, moving particle method,or the Doppler method as described below. The system of the inventionincludes a range phase cancellation memory for storage of a time-delayedversion of the background or baseline noise.

The system and method of the invention employ a wider dynamic range ofecho detection than has been employed in prior art non-coherent A-modesystems. Echoes therefore can be processed to yield a broader range ofamplitude and phase information, range time delay information, cloudlayer activity information, as well as particle speed information, suchas settling rate.

In another aspect, the system and method of the invention can beemployed to monitor settler performance and conditions regardless of thepresence of a detectable bed level. In yet another aspect, the systemand method of the invention enables reliable detection andcharacterization of unsettled cloud properties such as internal particleactivity and cloud thickness from the liquid level to the bottom of thesettler. A multiplicity of unsettled clouds therefore can besimultaneously detected and characterized. The results can be employedas an indicator of a settler upset condition.

Having briefly summarized the invention, the invention will now bedescribed in detail by reference to the following specification andnon-limiting examples. As used herein, "settler" includes devices suchas clarifiers, thickeners or other similar apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a schematic of the technique for measurement of bedlevel. FIGS. 1A and 1B also show plots of unipolar echo amplitude vs.depth within the settling tank.

FIG. 2A shows a schematic of echo detection at various points in aslurry.

FIGS. 2B-2D represent detected echo signals at various points shown inFIG. 2A.

FIGS. 3A-3C show the effects of processing of echo signals by theinvention.

FIG. 4 is a block diagram of the system of the invention.

FIG. 5 is a schematic of a bed level control process.

DETAILED DESCRIPTION OF THE INVENTION

The bed level of settled particles may be defined as the level where thedownward speed of settling particles is less than a given value, forexample 2"/min., which is near zero speed, or where a distinct,detectable boundary exists between the unsettled particle cloud and thesettled bed.

FIG. 1A shows a well defined bed level where an unsettled particle cloudis near the bottom of a settling tank but does not overlap the bedlevel. Echoes from the bed level therefore are clear and distinct. Theamplitude of the echo signals from the bed level also are shown in FIG.1A. In contrast, and as shown in FIG. 1B, when an unsettled particlecloud overlaps the bed level, echoes from the bed level are disrupted byechoes from the particle cloud. As a result, and as is shown in FIG. 1B,echoes from the bed level are blurred.

In accordance with the invention, and as shown in FIG. 2A, an ultrasoundtransducer T transmits an ultrasonic pulse into particle cloud 5 withina settling tank. Echoes from the particles generated by the ultrasonicpulse are reflected back toward the transducer and fed to thepreamp-receiver P-R when the ultrasonic pulse encounters particleclouds, bed level or settler bottom. The particle size of the materialsundergoing treatment in settlers can vary from process to process. Formaterials of about 500 to 3000 microns in diameter, ultrasonic wavefrequencies of about 200 to 500 KHz enable the detection of the bedlevel and "activity" or "cloud" level of unsettled particles with a highsignal to noise ratio. For materials of about 200 to 500 microns indiameter, ultrasonic wave frequencies of about 500 KHz to 2.25 MHz alsoenable the detection of the bed level and "activity" or "cloud" levelwith an equivalent signal to noise ratio. For smaller particle sizedmaterials undergoing treatment in a settler, for example, 50 to 200microns, the ultrasonic wave frequency should be increased to 2.25 to 5MHz for an equivalent signal to noise ratio.

FIG. 2B-D present signals generated by the Doppler method as well as themoving particle detection method employed in the invention for pointsP1, P2 and P3 in FIG. 2A. Point P3 in FIG. 2A represents the settled bedlevel at the bottom of a settler tank. Point P2 represents a position incloud 5 slightly above the surface of the settled bed level, and pointP1 represents a position within particle cloud 5. At points P1 and P2,echo signals vary with time about a zero baseline. The rate of variancedepends upon the speed of the particles, the transducer frequency andthe sound speed in the slurry. Typical waveforms generated from theseparticles are shown in FIGS. 2B and 2D. These waveforms can be processedby known Fast Fourier Transform (FFT) in the Doppler method and themoving target detection method to locate bed level. These data are usedto estimate a bed level when a distinct bed level boundary is notpresent. In contrast, signals from the nearly stationary bed levelboundary at point P3 yield a constant or nearly constant signal as shownin FIG. 2C. The waveform in FIG. 2C, when analyzed by FFT or movingtarget detection mode give a zero or near zero particle speed.

In each of the Doppler, peak and moving target coherent detection modesof the invention, sine (sin) and cosine (cos) signals are multipliedwith the echo analog signals. The amplitude and the envelope offrequencies of the resulting analog signals in each of the sine and coschannels represents echo data obtained along the path of the ultrasoundfrom the transducer. FIG. 3A shows analog signals generated by echoesfrom a distinct, slowly undulating bed level with no unsettled particlecloud overlapping the bed level. FIG. 3B presents amplitude andfrequency of echo data obtained from an unsettled cloud layer thatextends along substantially the entire transducer path to the settlerbottom. FIG. 3C shows amplitude and frequency of echo data obtained froma single distinct stationary bed level. Echo amplitude data at varyingdepths in the tank can be calculated from the sin and cos channel dataas described below.

Referring to FIG. 4, a block diagram of the system of the invention isshown. As shown in FIG. 4, a commercially available crystal oscillator,which is an element of the master oscillator, controls the time andlogic functions of the system. Useful crystal oscillators are availablefrom Digikey Co. of Minnesota. A frequency resonant crystal in themaster oscillator provides a stable basic frequency for the system. Themaster oscillator and logic also generate sin and cos frequencies formultiplying with the echo signals to generate new analog signals in themixers. These sin and cos frequencies are coherent with the transducerultrasound frequency and differ from each other by a 90° phase shift. Atime gated sin channel pulse signal, generated by the master oscillatorand amplified by the pulser, serves as the transducer excitation signal.All of these signals are coherent with the crystal oscillator. Asynchronous binary counter chain (not shown but part of the masteroscillator block) divides the crystal oscillator frequency to provide adesired transducer repetition frequency. For example, a transducerrepetition frequency for use in a 10 ft deep settler may employ a 0.003second delay between successive transducer pulses. The time delaypermits all echoes returning from a first transducer ultrasound pulse tobe received by a preamp-receiver before the next transducer pulse istransmitted.

Echo signals from the sin/cos channel mixers are filtered to removehigher order frequencies, forwarded to an amplifier, and then sent to ananalog-to-digital converter (ADC) via an analog switch. The ADCalternately samples the analog signals from sine and cos channels viathe analog switch for conversion into digital code. The resultingdigital code is sent to the Data Acquisition Memory. All timing andlogic signals to accomplish the analog switch selection,analog-to-digital conversion and storage in data acquisition memory aregenerated in the Master Oscillator Block, as is recognized by the artskilled. This enables the system to be synchronized with the mastercrystal oscillator and the system to be coherent.

Echoes are detected and amplified in the preamp-receiver block. Afeature of the preamp-receiver is the provision for time gain controlvoltage adjustment. The time gain control voltage can be shaped toincrease the preamp gain to compensate for transducer ultrasound pathspreading as the ultrasound pulse travels to the settler bottom. Echoamplitudes for all targets along the transducer beam path therefore tendto be equalized, therefore providing more reliable analog echo data.

Amplified echo signals from the preamp-receiver are transmitted to sinand cos mixers where they are multiplied by the sin and cos signals fromthe master oscillator. The analog signal output of each mixer consistsof signals with the echo amplitude having a frequency equal to thedifference between the transducer excitation frequency and the returnedecho frequency, plus higher frequency terms. The higher frequency termsare eliminated by filters following the mixers as shown in FIG. 4. Thesin/cos analog signal outputs from the filters can be increased in anamplifier for input to an analog-digital converter for conversion intodigital electrical signals.

Echoes returning from a stationary target such as a settled bed levelhave the same frequency as the original transducer ultrasound pulse,whereas echoes from moving targets such as particles are shiftedfrequency equal to the Doppler shift frequency. The Doppler shiftfrequency is estimated by the equation

    f.sub.d =(2vf.sub.t /c)*cos (φ)

where

f_(d) =Doppler shift frequency,

v=the velocity of particles,

f_(t) =transducer frequency,

c=speed of sound, and

φ=the angle between the direction of the particles and the transducerultrasound path. For example, if particles are flowing in a verticaldirection, either up or down, then φ=0 or 180° and cos (φ)=±1.

If an echo returns from a stationary target such as a settled bed level,the sin/cos channel analog signal outputs appear as in FIG. 3C. As shownin FIG. 3C, the frequency difference between the transducer ultrasoundfrequency and the echo frequency is approximately zero.

The polarity of the sin or cos analog signal depends upon therelationship between the phase of the sin/cos signal and the phase ofthe echo. For example, a unipolar amplitude signal from a clearlydefined bed level, such as shown in FIG. 1A, is found by calculating thesquare root of the sum of the square of the sin channel plus the squareof the cos channel.

Slowly moving isolated target echoes, such as echoes from a slowlymoving bed level boundary, appear as in FIG. 3A. As also shown if FIG.3A, the sin/cos analog output signals oscillate about the baseline at afrequency equal to the difference between the echo frequency and thetransmitted pulse frequency. A unipolar analog signal as shown in FIG.1B is obtained by taking the square root of the sum of the squares ofthe sin and cos analog signals.

Echoes from an unsettled cloud appear as in FIG. 3B. The oscillationfrequency of the echoes about the baseline at any point along thetransducer ultrasound path depends upon the frequency difference betweenthe returned echoes and the transmitted pulse frequency at that point.Generally, sin/cos analog signal outputs for unsettled cloud layers arelower in amplitude than for settled bed layers.

Background noise echoes may interfere with the desired echoes from theunsettled cloud particles by introducing a spurious range phase. Thisspurious range phase can be identified by directing an ultrasonic pulsefrom the transducer away from the bottom of the tank to generate echoeswhich represent the background noise and for storage in the range phasememories. During transducer pulses, the spurious range phase data isread from the range phase cancellation memory and coherently subtractedfrom the echo signals to eliminate the spurious range phase data. Therange phase cancellation memory requires periodic updating to accountfor varying settler conditions. The frequency of updating can be readilydetermined by the art skilled according to settler conditions.

Data collection variables and modes are established by operator input tothe computer. The operator enters data for the settler depth, transducerrepetition rate and the desired mode of operation, i.e., peak mode andmoving particle mode, and the Doppler mode. If the Doppler mode isselected, the operator enters the number and depth location(s) of theDoppler samples. Conveniently, any of the peak method, moving particlemethod and the Doppler methods can be employed, as described below.

A computer summarizes operator input data and stores that data in a fileon hard disk for ADC start, choice of image or Doppler modes, etc. Atstart of operation, this memory information is read into the systemlogic for preselected depths of the transducer for the Doppler mode. Allsubsequent logic and timing signals for system operation are derived inthe master oscillator block. These subsequent signals include ADC timingpulses, memory addresses and write pulses, as well as analog switchcontrol signal.

As mentioned, settler conditions are transient. The bed level boundarytherefore may not be in a satisfactory position for detection at thetime of a given transducer pulse. Thus, at a specific moment, a distinctbed level perpendicular to the transducer ultrasound path may exist, butat the next instant, the bed level may become less distinct or is nolonger perpendicular to the transducer ultrasound path. Advantageously,in the two image modes, echoes can be collected over several transducerpulses and stored in data acquisition memory. Data from the severaltransducer pulses increases the observation time of the bed levelboundary, if it exists, and the probability of bed level detection.

Data in the data acquisition memory can be processed by three distinctlydifferent methods as stated above. In the peak detection method, analogsine and cos signals are recalled from the memory on a transducerline-by-transducer line basis. As used herein, a transducer line is thecollection of echoes returning from a single ultrasound pulse.

Advantageously, the peak method enables bed level detection even if onlyone transducer line in the data acquisition memory detected a bed level.In addition, if the settled bed level is detected by several transducerlines in the data acquisition memory, then the probability of detectinga bed level is increased. The resulting line shows the maximum echopoint for each range increment on the transducer ultrasound path andpresents echoes from the settled bed level to determine the bed level.

To initialize the system for use with the peak method, the initial valuein each data point on the result lines for each of sin analog signal andcos analog signal is set to zero. As used herein, the result line is theline that presents the highest signal values at each depth within thesettling tank from all recorded transducer lines in each sin and coschannel. For each transducer line, the signal value at each depth in thesettling tank is compared to the value already present on the resultline at that depth to the value at that corresponding depth from the sinand cos channels. Then, the greater of the two signal values is assignedto the result line. Of course, the first transducer line will be greaterthan the initialized values in the result line. This process is repeatedover the number of transducer lines recorded in memory. The sin and coschannel result lines are equal to the depth by depth maximum value fromthe number of transducer lines.

The peak detection method provides a combined result line that is equalto the square root of the square of the sin channel data plus the squareof the cos channel data on a point-by-point basis along each of theprevious result lines. The bed level is found by locating the peakamplitude on the combined result line. When the bed level echo data hasa random character, whether during the sum of the periods fortransmitting the desired number of ultrasound pulses, i.e., theinterrogation period, or fragmented across that same interrogationperiod, the peak method significantly increases the probability of bedlevel detection by selecting the peak value for each result line datapoint.

In the Peak Detection method, the bed level and the activity level ofunsettled particles can be found by analyzing the resultant transducerline ultrasound data, Neural Network is the preferred method ofanalysis. Any of the available Neural Network software developmentpackages can be used in our invention. By presenting resultanttransducer lines of ultrasound data as the inputs and the correspondingmeasured real-time bed level and particle activity levels as theoutputs, a neural network is constructed that "learns" from the measureddata sets of inputs and outputs supplied to it. The resultant Neural Netdeveloped model is then used in the control part of our invention.

The moving target method may be used when the peak detection methodcannot find a settled bed level echo or when the bed level echoes aretoo near the background noise levels to be reliable. These conditionsmay exist when the particles may not be settling well, if at all, suchas during an upset condition where one or more particle clouds can form.Within these clouds, the unsettled particles move with different speeds.Under these conditions, identification of the number of clouds, theparticle activity within the clouds, and the size and position of theclouds is useful to describe the condition of the settler.

The moving target method recognizes that the moving particles changeposition between transducer pulses, thereby causing changes in the sineand cos analog signals. The moving target method estimates the particlemotion by subtracting one transducer line of sin and cos analog signalsfrom the next transducer line of data of sin and cos analog signals, andthen taking the absolute value of that difference. The absolute valuesof differences are summed over the number of transducer lines. This sumrepresents an integrated score of particle activity from the transducerto the bottom of the settler. An integrated particle activity scorethrough the depth of the settler is found by calculating the square rootof the sum of square of the sin analog signal plus the square of the cosanalog signal. Higher amplitudes indicate greater particle activity.

Clouds are found by the moving target method by identifying unsettledparticles with an integrated (sum) activity score above a baselinevalue. The baseline value can be determined by plotting the integratedscore of particle activity as a function of depth in the settler. Theintegrated particle activity scores for clouds are significantly higherthan that for a column of liquid that is cloud-free. The number ofoccurrences where the integrated scores deviate from the baselineequates to the number of clouds present in the tank. Further, thelocations of these deviations also indicate the locations of the cloud.Integrated activity scores thus can be used to identify the number ofclouds, the particle activity within an individual cloud, and the extentof cloud formation to evaluate the status of the settler regardless ofwhether a peak method determined bed level is obtained. For example, ahigh activity score indicates a cloud with a high degree of individualparticle activity. Large clouds present particle activity over a largerange, while small clouds present particle activity over limited ranges.Cloud formation provides data for evaluating settler condition, and theneed to adjust the amount and type of chemical additions, slurryadditions, settler rake speed, etc. in the settler.

In certain situations, a bed level is not sufficiently compact for thepeak method to detect. Such a bed level exists when there is a change inparticle density or a change in particle motion. This type of bed levelcan be called a soft bed level. The integrated activity scores may beused to identify a soft bed level. When one exists and there is nooverlapping cloud, the soft bed level will cause a sharper deviationfrom the baseline relative to a deviation caused by the cloud layer.When a cloud overlaps a bed, there will be no sharp deviation, but a bedlevel may be detected if the deviation caused by the cloud sharply orabruptly returns to the baseline. At the bed level, there is minimalparticle movement. As a result, the integrated activity score is nearthe baseline.

In the Doppler mode, particle speed in ascending and descendingdirections is calculated at specific points along the path of thetransducer's ultrasound path. According to the well known range equationabove, a depth in the settler is selected and the time T_(o) at whichthe ultrasound signal is transmitted is recorded. At a predeterminedtime T_(f), the frequency and amplitude of particle echoes is recorded.Time T_(f) is preset to correspond to a particular depth in the slurrybased on the speed of sound in the liquid. By varying T_(f), thesettling rate of particles at various distances from the transducer faceinto the tank can be monitored. Measurements at different depths can beobtained under computer control to construct settling rate profiles as afunction of time by FFT.

In the Doppler mode, the system of the invention can accommodate anumber of sample points, such as P₁ and P₂ in FIG. 2A. Conveniently,eight sampling points may be used. Only one data point in the sin andcos channels need be taken at each sample point for each line oftransducer data. After data acquisition is completed, the sin and cosanalog signal for each sample point are retrieved from the dataacquisition memory. These sin and cos analog data are analyzed usingFast Fourier Transform (FFT) or other frequency spectral techniques orspectral analyzer algorithm, such as autoregressive techniques.

The sampling time period or interrogation period required for use withthe Doppler method is determined by the minimum particle velocity to bedetected. In a settler, minimum particle velocities are on the order of1-2 inches/minute. The sampling time period therefore is (1/f_(d)),where f_(d) is the Doppler frequency corresponding to the minimumparticle velocity. Low velocities correspond to low frequencies in theFFT. Observation times in the order of a 10 seconds may be required toachieve this degree of resolution of particle speed by FFT. The minimumtransducer repetition frequency, however, is determined by the maximumparticle velocity expected. For example, using a 500 KHz transducer anda maximum particle velocity of 0.5 ft/sec, the Doppler frequency isabout 152 Hz, requiring a transducer repetition frequency of at least304 Hz.

In the Doppler mode, the range for a Doppler sample should be within thetransducer repetition frequency range. In addition, limited lengthwaveguide or a `quiet box` can be employed. The waveguide serves as achamber where the settler circulating currents introduced by therotating rake and the inflow from the center well are significantlyattenuated. Without the waveguide, the circulating currents cause theparticles to move in directions other than the vertical, making theDoppler speed and direction information difficult to interpret. Dopplerinformation from a waveguide corresponds closely with the sedimentationrates measured manually using a graduated cylinder outside the setterenvironment.

In an alternative embodiment, the settled bed level and the particleclouds may be measured at two or more sites within a settling tank bypositioning additional transducers further from the center well of thetank than the first, primary transducer. Multiple transducers may beused to maintain a high signal to noise ratio in bed level echoes toovercome problems of reduced signal to noise ratio in bed level echoesdue to bubbles. If bubbles adhere to a sufficiently large area of thetransducer face, the transducer will not operate efficiently, if at all.If bubbles accumulate on either of the transducers, the bubbles can beremoved by a wiper that passes under the transducer. Generation ofbubbles can be a significant problem in an aluminum clarifier. Bubblesmay arise when liquids are heated to temperatures near or above boiling.Bubbles also can arise when the liquid slurry is released from a highpressure source into a settler. A secondary transducer normally is freefrom bubbles unless the entire clarifier contains boiling liquid.

Another mode of the present invention involves the use of multipletransducers in arrays. A transducer array consists of one or moreindividual transducers or transducer elements. These transducer ortransducer elements are arranged geometrically to produce improvedultrasound beam characteristics over a single transducer or transducerelement.

The ultrasound beam generated has a greater sound intensity (power) andpressure than that of a single transducer along the path to the settlerbottom. The losses from the media, or liquid slurry, due to sound poweror pressure level sound saturation in the media are thereby reduced.Provided is a larger effective transducer aperture of the ultrasoundsource. The ultrasound beam divergence angle is determined by theeffective transducer geometry and the operating frequency. By using anarray, the effective aperture, i.e., area of the transducer array, isincreased. Increasing the aperture decreases the divergence angle of theresulting ultrasound beam. In addition, by summing the received echoesfrom each transducer element in the preamp/receiver, the beam divergenceangle is effectively decreased again.

Overall system sensitivity is increased by these signal processingtechniques. In the transit mode, the overall ultrasound pressure levelis increased. The sound pressure level is the sum of the pressure levelsproduced by each transducer, provided the pressure level is below thesaturation level of the media. The result is that the overall echoreturn sensitivity and strength is increased, improving theidentification of the bed level and unsettled particle "activity" or"cloud" level return echoes in the settler.

The arrays used in this invention may consist of, for example, two tofour individual transducers, spaced in such configuration as a triangleor a square. One to three arrays may be spaced along the slurry orsettler radius. The spacing and number of transducers may also change,depending on the slurry or settler depth and the media in the settler.

As stated above, a waveguide or a "quiet box" may be positioned in asettler. The waveguide including the transducer extends into the settlersuspension. The waveguide attenuates the effects of random circulatingcurrents which may be present in the settler to enhance measurement ofthe downward movement of the settling particles.

The waveguide can be a cylinder with slots therein to allow a liquidslurry to enter the waveguide. Waveguides are made from materialscapable of withstanding temperatures above 100° C. and corrosive liquidsand which do not bend under currents which might misdirect theultrasound beam.

The waveguide causes the transducer beam to approach a plane wave thatremains essentially constant over the length of the waveguide to reducebeam spreading to minimize any significant decrease of echo amplitudeand detection reliability. The waveguide also attenuates circulatingcurrents present in the settler. Attenuation of circulating currentsprovides particle vertical settling rates which correlate well withrates obtained manually with a suspension in a graduated cylinderoutside the settler and a stopwatch.

Positioning a transducer at the top end of the waveguide effectivelyincreases the diameter of transducer to that of the waveguide at thebottom end, and also effectively positions the transducer closer to thebed level particle interface by a distance equal to the length of thewaveguide. A further advantage of positioning a transducer at the topend of a waveguide is that measurement of sample volume required toobtain settling rate data can be taken electronically at variouspositions in the waveguide.

As mentioned, settling process conditions within a tank can betransient. It therefore is important to avoid upset conditions which maydisrupt the settling process conditions. Advantageously, the particlesettling rate and the position of the settled bed level obtained by thesystem and method of the invention can be employed to control the rateof addition of chemical additives and particle slurry to the tank.

As shown in FIG. 5, the bed level and settling rate can used to controlthe rate of addition of chemicals 1 and 2 via valves CV1 and CV2,respectively, to a slurry of solids 1 added to settling process tank viathe flume to cause solids to precipitate and to thereby recoversubstantially clear liquid. As shown in FIG. 5, precipitated wastesolids are drawn from the settling tank via pump PU2 and residual liquidis recirculated to the slurry.

The resultant neural network software developed model takes thereal-time ultrasound measurements and calculates the corresponding bedlevel and activity level inputs to a Fuzzy Logic controller. The FuzzyLogic controller calculates the outputs to valves CV1 and CV2 (FIG. 5),controlling the addition of chemicals 1 and 2 to give the desired bedlevel and activity level set points to control the dynamic conditions ofthe settler. This Fuzzy Logic controller can be based on any of thecommercially available Fuzzy Logic software control packages, with, forexample, the neural net model presenting inputs to the Fuzzy Logiccontroller, with object codes for both the Fuzzy Logic and Neural Netprogrammed in C, FORTRAN, or any other transportable coding system. TheFuzzy Logic controller consists of a series of rules that governs theaddition of chemicals 1 and 2, and any other additional chemicals,and/or other control parameters of the settler, such as pump PU2 of FIG.5, controlling the underflow pump out rate, thus controlling the settlerset points.

What is claimed is:
 1. A method for identifying an unsettled particlecloud within a settling tank comprising,transmitting ultrasonic soundwaves emanating from an array of 2 to 4 transducers into a slurry ofparticles to generate echoes from said particles; generating electricalsignals from said echoes; dividing said electrical signals into sine andcosine channels; multiplying the electrical signals in the sine channelby a sine signal of the same frequency as the ultrasound sound waves toprovide a sine channel result; multiplying the electrical signals in thecosine channel by a cosine signal of the same frequency as theultrasound sound waves to provide a cosine channel result; convertingthe electrical signals in said channels to digital signals; storing thedigital signals according to transducer lines in memory; subtracting thedigital signals from each transducer line from a succeeding transducerline on a depth-by-depth basis; summing absolute values of differencesbetween the transducer lines in each channel; and constructing acombined result line from said absolute values by determining the squareroot of the sum of the squares of sine channel results and the squaresof cosine channel results; and locating particle clouds by identifyingdeviations from a baseline of the combined result line.
 2. A method ofclaim 1 wherein one or more ultrasound transducer arrays are spacedalong the slurry radius.
 3. A method of claim 1 wherein the ultrasoundtransducer array is in the form of a triangle.
 4. A method of claim 1wherein the ultrasound transducer array is in the form of a square.